The need to measure surface geometry and position of objects for part setup, refixturing, digitizing (i.e., converting a part model into a numerical description), machining, inspection and qualification has resulted in the development of probes and probe routing systems and techniques to automatically scan the surface of an object. These are often used in connection with multipurpose manufacturing equipment such as computer numerically controlled (CNC) machine tools or with dedicated manufacturing equipment such as coordinate measuring machines (CMM). Surface data is obtained by moving a probe relative to the surface being inspected along three orthogonal axes under computer control and sampling both the probe output and probe position data at regular intervals. The samples are used to generate a profile of the part surface, and may also be used to issue probe-routing signals to control the probe trajectory.
Probes used in such systems may be characterized as either digital or analog. Digital probes, also known as on/off contact probes, provide discrete output signals indicating whether or not a threshold value associated with the probe has been crossed; when the threshold value is exceeded, the probe is said to be "triggered." Most digital probes utilize a threshold based on a position displacement (so-called "touch trigger probes"), but force and pressure effects may also be used.
Touch trigger probes have a stylus connected to a sensitive switch which operates when the stylus is displaced from its non-triggered to its triggered position. Because the control system will generally not be able to stop probe fixture motion instantaneously upon contact of the probe with the part surface, the probe is designed to be capable of a certain degree of overtravel beyond the triggered position. The maximum allowable displacement of the device (i.e. the permitted limit of overtravel) is often referred to as its safe operating distance or safe operating range.
A data gathering cycle is typically initiated with the probe in its non-triggered or rest state. The main processor issues control signals to the probe positioning driver subsystem instructing it to decrease the distance between the surface to be measured and the probe. Motion continues until the probe contacts the surface. The triggered state signal from the probe is then used to stop motion of the probe. The control system then signals the positioning driver to return the probe to its rest position and the data gathering process repeats itself.
The change in status of the probe from the non-triggered to the triggered state and position feedback from the probe positioning driver is used by the control system to generate sampled data point in three dimensions. A complete set of data points representing the surface of the part is created as the probe is routed along the part surfaces by either moving the probe along a pre-determined path or by computing the path based on probe output data. One device of the latter type is shown in Hong et al. U.S. Pat. No. 5,208,763.
The iterative cycle of positioning, moving, stopping and sampling, as practiced by the prior art, can be quite time consuming for a large or complex part, as presently available equipment typically can generate only 1-2 data points per second--or in the newest systems, 4-5 points per second. Thus, digital probing systems have been impractical up to now for rapid collection of large data sets.
To address this deficiency of digital probes, analog probes were developed. Analog probe systems provide continuous sensor feedback signals indicating the instantaneous magnitude and direction of the displacement, force, pressure or other measurable effects. The control system uses the sensor feedback to issue control signals to move the probe along a path which keeps the sensor feedback within a predefined operating range. While motion is taking place, the control system combines instantaneous probe position feedback data with the instantaneous analog sensor feedback data to generate the sampled data points. This allows sampled data points to be generated without the need to stop and initialize the probe between readings.
Unfortunately, the increased sophistication of analog probes over their digital counterparts makes them substantially more costly. Also, much more sophisticated electronics is required to coordinate the sampling times between the sensor feedback and the probe driver. As a result, analog probing systems are much more expensive than digital systems, and even though they are much faster, have proved to be of limited utility. It is clear that there would be substantial advantages to a digital probing system having the data gathering speeds of the much more expensive analog systems.
Accordingly, it is an object of this invention to provide improved methods and equipment for collection of surface configuration and position data, and more particularly, from part models, templates or the like in connection with computer controlled design and manufacturing.
It is also an object of this invention to provide such improved methods and equipment using digital touch trigger probes.
It is a further object to provide data collection capability using digital touch trigger probes which could only be achieved up to now using analog probes.
It is a further object of this invention to provide a method and system for routing a digital data gathering device which may be utilized to automatically obtain surface data from physical objects with speed and efficiency previously attainable only through use of much more expensive analog devices.